ALCOHOL-FREE beer may seem a contradiction in terms. And if the lack of bite
doesn鈥檛 get to you, the nasty taste will鈥攁s any pregnant woman or
designated driver will attest. Now Belgian scientists have identified the
chemical to blame, and they鈥檇 be half-way to solving the problem, but for public
mistrust of genetic engineering.
Real beer contains between 3 and 12 per cent ethanol, while any brew with
less than 1 per cent can be classified as 鈥渁lcohol-free鈥�. All beers start out as
wort, a mixture of ground, malted grain and water, which is then fermented with
yeast. This creates both the alcohol and the mixture of esters, sugars and
complex alcohols that give a beer its unique taste and aroma.
Most low-alcohol beers are fermented at 1 掳C, to keep the yeast from
making much alcohol. But this also stops the flavourful chemicals from
developing, says Philippe Perp猫te of the Brewing and Food Industries
Laboratory of the Catholic University of Louvain. 鈥淭he worty, `off鈥� flavour of
alcohol-free beer comes from chemicals that would ordinarily have been
transformed by the yeast,鈥� he says.
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His team analysed the chemical make-up of alcohol-free beer, ordinary lager
and wort by passing them through a gas chromatograph. This separates the
compounds, letting researchers smell each one as it travels past a 鈥渟niffing
辫辞谤迟鈥�.
Four samplers took it in 20-minute turns to describe the scents that wafted
by. 鈥淥ne person can鈥檛 do the whole thing,鈥� says Perp猫te. 鈥淵ou can鈥檛 smell
anything after a while.鈥� Using mass spectrometry to identify the chemicals
behind the smells, they discovered a particularly unpleasant one was
3-methylthiopropionaldehyde, or methional. Its scent persisted at low dilutions,
and was especially strong in both wort and alcohol-free beer, the authors report
in the Journal of Agricultural and Food Chemistry (vol 47, p 2374).
Further experiments showed that the methional in wort is largely broken down
by subsequent fermentation. What little is left in normal beer is masked by
other flavours, says Perp猫te. Even the few alcohol-free beers that are
made by evaporating alcohol from normal beer suffer from the 鈥渙ff鈥� taste, he
says, because the chemicals that usually mask it are removed with the
alcohol.
At a meeting of the American Society of Brewing Chemists in Phoenix, Arizona,
later this month, Perp猫te鈥檚 team will announce their bid to solve the
problem: an artificial yeast in which the gene for one enzyme, an alcohol
dehydrogenase, has been snipped out. The yeast needs the enzyme to make ethanol,
but it also produces methional. Perp猫te suspects the modified yeast would
be perfect for alcohol-free beer, but the food industry in Europe is nervous
about genetic engineering. 鈥淣o brewer is ready to use it.鈥�